SPECT is one of Nuclear Medicine imaging techniques that enable to acquire functional information about patient's specific organ or body system. This functional information is attained from analysis of internal radiation obtained from pharmaceutical substance administered to the patient, which is labeled with a radioactive isotope. The radioactive isotope decays, resulting in the emission of gamma rays, thus providing with information on the concentration of the radiopharmaceutical substance introduced to the patient's body. An instrument for the detection of gamma ray emissions of the radiopharmaceutical substance administered in the body is known as gamma camera. The SPECT technique collects gamma ray photons that are emitted from the patient and reconstructs an image or a series of images of the place in the body from which the gamma rays are originated. From this picture a physician can determine how a particular organ or system is functioning. The main components making up a conventional gamma camera are a collimator for passing therethrough gamma rays to be detected, photon detector crystal or detector array, position logic circuits and data analysis computer. Depending on the type of the detector crystal, conventional gamma camera may or may not include a photo-multiplier tube array.
A gamma ray photon that has passed through the collimators, interacts with the detector crystal by means of the Photoelectric Effect or Compton Scattering with ions of the crystal. These interactions cause the release of electrons which in turn interact with the crystal lattice to produce light, in a process known as scintillation. Since only a very small amount of the light is given off from the crystal, photo-multiplier tubes are normally attached to the back of the crystal. Typically, such conventional gamma camera has several photo-multiplier tubes arranged in a geometrical array. The position logic circuits that follow the photo-multiplier tube array, receive the electrical impulses from the tubes and determine where each scintillation event occurred in the detector crystal. Finally, in order to deal with the incoming projection data and to process it into a readable image of the spatial distribution of activity within the patient, a processing computer is used. The computer may use various different methods to reconstruct an image.
Different collimators are used in gamma cameras to limit the detection of photons to incidence range of predetermined angles. A parallel-hole collimator is usually made from lead or tungsten and has thousands of straight parallel holes in it, allowing only those gamma rays traveling along certain directions to reach the detector. As a result of that, the ratio of emitted, versus detected, photons may reach 10000 to 1. In order to decrease this ratio, converging or diverging hole collimators, for example, fan-beam and cone-beam are also known in the art. The usage of these collimators increases the number of photon counts, which improves sensitivity. The sensitivity, however, is inversely related to geometric resolution, which means that improving collimator resolution decreases collimator sensitivity, and vice versa.
The current use of collimators results in a rather low detection efficiency of conventional SPECT which leads to prolonged data acquisition time and the need to administer high dosage of the radiopharmaceutical substance.